In a medical field such as medical science and dentistry, the first choice as a means of restoring a large bone defect or void is autogenous bone grafting onto such a defect or void. However, grafting using bone graft material that can be substituted for an autogenous bone is widely done because invasive surgery has to be done on a healthy part in order to take an autogenous bone, and there is a limit on the amount of a bone to be taken. Mechanical characteristics, biosafety, osteogenic potential and so on, which are similar to those of a in vivo bone are required from this bone graft material.
Bone graft material is categorized as: an allogeneic bone taken out of a dead body; a xenogeneic bone taken out of another animal such as cattle; and a chemically synthesized artificial bone. While an allogeneic bone and a xenogeneic bone may carry a risk of infectious diseases due to contamination by factors originated from another organism, an artificial bone does not carry such a risk, which is superior. Thus, artificial bones have been developed in recent years.
A ceramic artificial bone whose main component is calcium phosphate is known as an artificial bone. Material most studied is hydroxyapatite. Hydroxyapatite is an extremely useful bone graft material because of its osteoconductivity. However, hydroxyapatite is non-bioresorbable material and does not disappear. Thus, hydroxyapatite remains in a body as a foreign body semipermanently. This might cause leakage from a grafted defect, and inflammation due to infection of a graft. Therefore, bioresorbable bone graft material is desired.
Therefore, a ceramic artificial bone consisting of β-tricalcium phosphate (β-TCP), which is bioresorbable material, has been developed (for example, see Patent Literature 1). This artificial bone is superior in bioresorbability, and thus, disappears in the end. However, the mechanism of its resorbence does not depend on that of a living body such as physicochemical solution. Thus, if a bone defect is large or the like, there is a possibility that an artificial bone disappears before the bone is sufficiently ossified.
In contrast, carbonate apatite has been developed in recent years as bone graft material resorbed according to a mechanism of a living body (for example, Patent Literature 2). Carbonate apatite has a composition similar to an in vivo bone, and thus, is resorbed according to a mechanism of a living body. Therefore, it is said that a bone can be repaired with high predictability because bone formation by osteoblast cells and resorbence of bone substitute material by osteoclast cells (remodeling) are properly carried out.
A method of immersing a precursor of calcium carbonate in a phosphate solution is effective as a method for manufacturing carbonate apatite (for example, Patent Literature 2 described above). Here, only carbonate apatite over a certain size, which can have, for example, a granular or block-like shape, can be applied to a living body because it is known that, a living body recognizes, for example, powdery bone graft material under a certain size as foreign bodies, to induce inflammation. On the other hand, for example, large block-shaped carbonate apatite is preferable because capable of a large bone defect or the like.
Large block-shaped calcium carbonate is necessary as a precursor in order to obtain, for example, large block-shaped carbonate apatite. However, calcium carbonate is powdery, and is necessary to be artificially shaped into a block. For example, sintering cannot be employed because calcium carbonate is decomposed if sintering is carried out thereon. Although there is some disclosure of shaping calcium carbonate into a block, such disclosure cannot be employed for a raw material of an artificial bone that requires biosafety. For example, an inorganic filler such as calcium carbonate are bound by an organic and/or inorganic binder to be hardened, to obtain a calcium carbonate block that is generally called cultured marble (for example, see Patent Literature 3). Such a calcium carbonate block cannot be employed because there is a possibility that impurities that might have a bad influence on a human body remain.
In contrast, in Patent Literature 2 described above, powder of calcium hydroxide is compression-molded and the resultant compressed body is subjected to carbonation under a stream of carbon dioxide with a relative humidity of 100% to obtain calcium carbonate blocks. According to this method, calcium carbonate blocks can be obtained without a problem about biosafety because safe calcium hydroxide according to Japanese Pharmacopoeia or the like is available, and the powders bond with each other at the same time of the carbonation, to give the blocks strength.